Method of making nickel powder



United States Patent Ofilice 3,367,757 Patented Feb. 6, 1968 3,367,767METHOD OF MAKING NICKEL POWDER DeWitt Henry West, Port Eynon, and DavidMyers Llewelyn, Clydach, Swansea, Wales, assignors to The InternationalNickel Company, Inc., New York, N.Y., a corporation of Delaware NDrawing. Filed Aug. 16, 1965, Ser. No. 480,106 Claims priority,application Great Britain, Aug. 19, 1964, 33,907/ 64 5 Claims. (Cl.75--.5)

ABSTRACT OF THE DISCLOSURE Carbonyl nickel powder with low carboncontents is produced by decomposing nickel carbonyl mixed with ammoniaand in the substantial absence of water vapor in a decomposer havinginner surfaces of nitrided steel.

This invention relates to carbonyl nickel powder, that is to say powdermade by the thermal decomposition of nickel carbonyl vapour in the hotfree space of a decomposer.

The production of carbonyl nickel powder in this way has been carriedout on an industrial scale for many years, and it is well-establishedthat according to the conditions of temperature, the concentration ofcarbonyl, and the presence or absence of diluent gases, e.g. carbonmonoxide, the power produced may assume one of two forms. These are theso-called A carbonyl nickel powder, which consists of discrete particleswith an irregular spiky surface, and the so-called B powder, whichconsists of agglomerates of interlocking filaments or chains ofinterconnected (aggregated) particles which again are individuallyirregular. Type B powder has a low bulk density and has a microscopicappearance of small spongy flakes. The size of the aggregates ofparticles making up the chains can vary widely.

Whatever its physical form, the powder product from the decomposerordinarily contains a small amount, e.g. from 0.05 to 0.08% by weight,of carbon. Most of the carbon is present in the nickel particles, eitherchemically combined with the nickel or as graphite. However, the productis almost always contaminated with a very small proportion, generallyless than 0.01% by weight, of particles of very high carbon content,e.g. 50% or more. For many uses of the powder the presence of thesecarbonaceous particles is most undesirable. Thus when the powder issintered in thin layers the carbon particles burn away leaving holes.When the thin layer is a protective coating on steel the protectionafforded is thereby reduced, and in the case of a porous product such asa fuel cell electrode, the pore distribution is rendered nonuniform andthey must therefore be removed from the powder before it is used. To theextent that the carbonaceous particles are larger than the nickelparticles this can be done by screening, but it is not always possibleto eliminate all of them in this way.

Although attempts have been made to avoid the formation of carbonaceousparticles during the production of carbonyl nickel powder and to obtaina product of lower carbon content, none, so far as we are aware, wasentirely successful when carried into practice commercially on anindustrial scale.

It has now been discovered that the formation of carbonaceous particlescan be prevented or reduced by a special conditioning pretreatment ofthe steel inner surfaces of the decomposer, and that the carbon contentof the particles of the nickel powder can be lowered by the use ofspecial additions to the decomposer.

It is an object of the invention to produce carbonyl nickel powdersubstantially free from carbonaceous particles, for use in powdermetallurgy.

Another object of the invention is to produce carbonyl nickel powder ofwhich the particles have a lower carbon content.

Other objects and advantages will become apparent from the followingdescription.

The present invention is based on the discovery that the carbonaceousparticles are formed by the thermal decomposition of carbon monoxide atthe surfaces of the steel walls and internal fittings of the decomposer,and that their formation can be wholly or partly prevented by nitridingthese surfaces.

We further find that both the formation of the carbonaceousparticles-and the incorporation of carbon into the nickel particles inother forms is promoted by water vapour. Based on these discoveries, theinvention consists in producing nickel powder by decomposing nickelcarbonyl vapour in the substantial absence of water vapour in the hotfree space of a decomposer whose steel inner surface have been nitrided.

The nitriding of the steel decomposer surfaces may conveniently beperformed by beating them in contact with ammonia gas, e.g. by leadingammonia gas into or through the decomposer while the walls and otherinternal surfaces are heated to a high enough temperature for a surfacelayer of iron nitride to be formed by reaction with the ammonia. Asuitable temperature is 500 C., and the heating should be continued forat least 1 hour, e.g. for 3 hours. The effectiveness of a nitridingtreatment may be assessed by simultaneously treating a test-piece ofsteel of similar composition in the decomposer and then exposing it at500 C. to carbon monoxide containing small amounts of nickel carbonylvapour, for two hours. If no black deposit of carbon is formed on thesteel surface, the treatment is eifective.

While it is preferred to perform the nitriding as a separatepre-treatment before introducing the carbonyl into the decomposer, itmay if desired be done by passing ammonia into the decomposer while aprevious batch of nickel powder is being made and the decomposer wallsare therefore heated. It will be appreciated that the full benefit ofthis treatment in preventing the formation of carbonaceous particleswill not be obtained until the decomposer walls have been fullynitrided. In whichever way the treatment is performed, the internalsurfaces of the decomposer must of course be initially clean.

Once the nitriding treatment is complete, it is still advantageous,according to a further feature of the invention, to mix a smallproportion of ammonia with the nickel carbonyl being decomposed. Thisboth serves to maintain the nitrided surface layer on the steel surfacesand also reduces the content of combined carbon in the powder formed. Webelieve that the latter efliect is due to inhibition of thedecomposition of carbon monoxide on the newly-formed surfaces of thenickel powder particles to form carbon that is normally incorporatedinto them. This effect is however counteracted] by water vapour, andcare should be taken that this is substantially absent.

A further slight reduction in the combined carbon content of the powderresults if a small proportion of oxygen, e.g. from 0.01 to 0.1% byvolume of the gas feed, is introduced during the decomposition as wellas the ammonia. The ratio of ammonia to oxygen is preferably 4:3 byvolume. Whether or not oxygen is introduced, the proportion of ammoniarequired is very small, as little as 0.01% by volume of the gas feed tothe decomposer generally being effective. To minimise contamination ofthe eflluent gases and avoid the formation of significant Before theseries of tests began, the internal surfaces of the steel decomposerwalls were clean and free from nitride films. The results in the tableshow that when no addition of ammonia or oxygen was made, the carbonamounts of water vapour by the reaction of ammonia 5 content of thepowder formed was 0.057% and the nitrowith oxygen, the proportion ofammonia preferably does gen content was negligible (Expt. 1). Thispowder connot exceed 0.1% by volume. The presence of oxygen protained asmall proportion of black carbonaceous particles. motes nitriding of thepowder owing to the formation of The walls of the decomposer were thennitrided by introactive nitrogen by reaction between the ammonia andducing 5 litres/hour (0.25% by volume) of ammonia gas oxygen, and thenitrogen content of the product is thereinto the decomposer feed for 3hours while nickel carbonyl fore increased. In any event the nitrogencontent of the was decomposed as before, and further runs (Expts. 2-16)powder must be less than 0.01%, since with 0.01% or were then carriedout without any additions, with ammore nitrogen the powder particlesbecome spherical, and monia additions and with additions of both ammoniaand the ammonia concentration must therefore not exceed oxygen. Care wastaken that all the added gases were dry. 0.2% by volume when oxygen ispresent. In all of Expts. 2 to 16 according to the invention the Theinvention is applicable to the production of nickel powder product wasfree from black carbonaceous partipowder by decomposing nickel carbonylvapour not only cles. The carbon contents of the powders made with aminconcentrated form but also when it is diluted with carmonia additionswere lower than those made under similar bon monoxide, which may bepresent in excess. It can be conditions without ammonia after thedecomposer walls used to make both type A and type B powder. The form 20had been nitrided, and those made in the presence of both of the powderparticles is not affected by the presence of ammonia and oxygen arelower still. The somewhat higher ammonia alone in the decomposer, butwhen both arncarbon contents of the powders made in Expts. 3 to 5 aremonia and oxygen are used and the nitrogen content apdue to the lowercarbonyl concentration. proaches 0.01% by weight, the irregularities onthe sur- The powders made using ammonia and Oxygen had face of thePflfticles are smoothed out and they tend to higher nitrogen contents,and particles of No. 5, which become spherlcal. contained 0.009%nitrogen, were almost spherical, with The operating conditions underwhich A and B type roughed surfaces. nickel powders are obtained in acarbonyl decomposer are The powder produced in all the experiments wasof now well understood. In general, conditions in which the Type A. Thatmade in accordance with the invention disrate of nucleation is high leadto the formation of type solved completely in hydrochloric acid to aclear solution B powder, while a low rate of nucleation leads to theforof nickel chloride, whereas the product from Expt. 1 left mation of Atype powder. A high rate of nucleation is an insoluble scum of carbon.favoured y high temperature, high Carbonyl cohcehtl'a' Although thepresent invention has been described in lion and high throughput, Whilea 10W nucleation rate is conjunction with preferred embodiments, it isto be underfavohred y low temperaturs, 10W carhohy1 Concentration stoodthat modifications and variations may be resorted to aRC1 10W throughputThesfi Same considerations apply without departing from the spirit andscope of the inven- When the decomposition is Carried out in thePresence of tion, as those skilled in the art will readily understand.ammonia. It should b n t however, that increasing the Such modificationsand variations are considered to be temperature increases the tendencyfor carbonaceous par- 40 within the purview and Scope f the inventionand holes to form and for carbon to be introduced 1nto the pended 1nickel particles. Preferably, therefore, the temperature We claim; useddoes not exceed about or 1. A process in which non spherical nickelpowder sub- The features of the process are illustrated by therestantially free from particles of carbon is produced by the sults of aseries of experiments in which nickel carbonyl decomposition of nickelcarbonyl vapour in the substanvapour diluted with about 11 times itsvolume of carbon tial absence of water vapour in the hot free space of amonoxide (nickel carbonyl concentration 69% by decomposer havingnitrided steel inner surfaces. volume) was fed into an externally-heated10-inch diam- 2. A process according to claim 1 in which ammonia eterdecomposer having mild steel walls at a total gas flow is admixed withthe nickel carbonyl vapour. rate of 2000 litres/hour through an inlet atthe top. Dry 3. A process according to claim 1 carried out in adeammonia gas and oxygen were injected into the inlet gas composer, thewalls of which have been nitrided during stream in the amounts indicatedin the table. The internal the decomposition of nickel carbonyl in thepresence of temperature, measured at a point half-way between theammonia in a previous operation. Wall and the axis and one-quarter ofthe distance down 4. A process for producing non-spherical nickel powderfrom the top, was 290 C. in all the tests, and the wall substantiallyfree from particles of carbon which comtemperature, measured by athermocouple strapped to the prises decomposing nickel carbonyl vapor inthe substaninside of the heated wall, was about 400 C. tial absence ofwater vapor in the hot free space of a TABLE E Feed rate (litres/hr.)Concentration (percent by volume) Properties of Powder Product NickelFisher Bulk Chemical Composition NH3 0; Carbonyl NH3 02 Value, Density,

microns glIDS./CG. Percent C Percent N 3 9 4. 47 2. 47 0. 057 0. 001 73.66 1. 99 0. 030 0. 001 7 0. 4 4. 2 2. 42 0. 035 0. 002 7 0. 0s 4. 362. a9 0. 035 0. 002 7 0. 2 6. 20 2. 87 0. 033 0. 009 9 0. 1 5. 02 a. 020. 025 0. 003 9 0. 05 6. 20 2.82 0. 029 0. 004 9 0. 025 6.26 2. 74 0.0280. 005 0 0. 0125 5.83 2. 73 0. 02s 0. 004 9 0. 0125 4. 5s 2. 41 0. 0220. 004 9 0. 025 0. 075 4. 2s 2. 1s 0. 020 0. 004 9 0. 05 0. 075 5.10 2.36 0. 025 0.004 9 0. 1 0. 075 4. 86 2. 25 0. 019 0. 000 0 0. 1 0. 03755. 5s 2. 92 0.020 0. 000 0 0.1 0. 010 5. 42 2. 48 0. 020 0.004 0 0.1 0.0005 5. 02 2. 91 0. 010 0. 003

5 6 decomposer having nitrided steel inner surfaces and ad- ReferencesCited mixing ammonia and oxygen with the nickel carbonyl UNITED STATESPATENTS vapor, said ammonia being added in amounts from about 0.01% to0.2% by volume of the gas feed and said oxygen 1759661 5/1930 Muller eta1 75 0'56 being added in amounts of from about 0.01% to 0.1% by 5HYLAND BIZOT, Primary Examiner. volume of the gas feed.

5. A process as described in claim 4 wherein the ratio DAVID RECKExammer of ammonia to oxygen is about 4 to 3 by volume. W. STALLARD,Assistant Examiner.

